On Tuesday, April 3, 2018 at 5:44:38 AM UTC-7, Dmitry Vyukov wrote:
>
> On Sat, Mar 31, 2018 at 10:41 PM, Chris M. Thomasson
> <cri...@charter.net <javascript:>> wrote:
> > Notice how there is an acquire barrier inside of the CAS loop within the
> > enqueue and dequeue functions of:
> >
> >
> >
> http://www.1024cores.net/home/lock-free-algorithms/queues/bounded-mpmc-queue
> >
> >
> > ?
> >
> >
> > Well, fwiw we can get rid of them by using stand alone fences:
> >
> >
> > Btw, sorry for the membar abstraction. I can quickly get sick and tired
> of
> > having to type out (std::memory_order_relaxed) all the damn time. ;^)
> >
> >
> > Anyway, here is the simple code using Relacy:
> >
> > ___________________
> [...]
> > ___________________
> >
> >
> >
> > There is no need for any membars within the loops. The version count
> keeps
> > everything in order.
>
> compare_exchange in C/C++ has optional failure memory ordering, so the
> following looks equivalent to me ;)
>
> m_tail.compare_exchange_weak(ver, ver + 1, acquire, relaxed)
>
Yes, you are correct about that. However, I personally like to keep
everything relaxed, and only add memory barriers when they are absolutely
needed. The stand alone fence can do this. Ahh, perhaps I am thinking too
much of SPARC with its awesome membar instruction. For instance, the
acquire barrier you have in the CAS loop in your MPMC queue code is what a
stand alone fence can get rid of. Think of the following line of code, that
is within the loop:
size_t seq =
cell->sequence_.load(std::memory_order_acquire);
Executing an acquire barrier on every iteration of the CAS loop is not
necessary. The actual version count keeps everything in order.
However, you do need a single acquire fence _after_ the CAS loop succeeds in
order to get a clear view of the element.
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